Thanks for spotting that Morrigan.
This works with both positive and negative number

=INDEX(LARGE(IF(A1:A20=0,"",A1:A20),ROW(INDIRECT("1:"&(COUNT(A1:A20)-COUNTIF(A1:A20,0))))),RANDBETWEEN(1,COUNT(A1:A20)-COUNTIF(A1:A20,0)))

However this formula MUST be confirmed by holding down Ctrl and Shift, and then hit Enter.

Ola Sandström